Field of the Invention
The invention relates to a method for the hot-dip coating with zinc or a zinc alloy of a flat steel product containing 2-35 wt. % of Mn and to a flat steel product provided with a coating of zinc or a zinc alloy.
Description of Related Art
In the modern-day automotive industry, increasing recourse is being had to high strength and very high strength steels. Typical alloying elements are, amongst others, manganese, chromium, silicon and aluminium which, when subjected to conventional recrystallisation annealing treatment, form stable, non-reducible oxides on the surface. These oxides may hamper reactive wetting by molten zinc.
Because of the beneficial combination of properties which they have, comprising on the one hand high strengths of up to 1,400 MPa and on the other hand extremely high elongations (uniform elongations of up to 70% and elongations at rupture of up to 90%), steels having high-manganese contents are, basically, particularly suitable for use in the field of vehicle construction and in particular automobile construction. Steels specifically suitable for this purpose having high Mn contents of 6 wt. % to 30 wt. % are known from, for example, DE 102 59 230 A1, DE 197 27 759 C2 or DE 199 00 199 A1. While being of high strength, flat products produced from known steels have isotropic behaviour when being formed and, what is more, are still ductile even at low temperatures.
However, counterbalancing these advantages is the fact that high-manganese steels tend to suffer pitting corrosion and are difficult to passivate. When there is an exposure to increased concentrations of chloride ions, this tendency to suffer corrosion which, though limited locally, is nevertheless severe, is high in comparison with less highly alloyed steels and it makes steels belonging to the group of high-alloy sheet steels difficult to use in the very field of bodywork construction. What is more, high-manganese steels also have a tendency to suffer surface corrosion, which is likewise a factor which limits the range over which they can be used.
It has therefore been proposed that flat steel products produced from high-manganese steels should also be provided in a manner known per se with a metallic coating which will protect the steel against corrosive attack. As well as revealing fundamental problems relating to wetting by the molten Zn, particularly with regard to the adhesion to the steel substrate which the coating is required to show during cold forming, practical attempts to provide steel strip containing high manganese contents with a metallic protective coating by hot-dip coating able to be carried out at low cost have failed to produce satisfactory results.
The reason for the poor adhesion properties was determined to be the thick layer of oxide which forms in the course of the annealing which is indispensable for the hot-dip coating. The surfaces of sheet metal which have oxidised in this way can no longer be wetted with the requisite uniformity and completeness by the coating metal, which means that the aim of corrosion protection covering the full area is not achieved.
Possible ways of improving wettability by applying an intermediate layer of Fe or Ni which were known from the field of high-alloy steels but ones having lower Mn contents failed to achieve the desired success with sheet steel containing at least 6 wt. % of manganese.
It has been proposed in DE 10 2005 008 410 B3 that a layer of aluminium be applied to steel strip containing 6-30 wt. % of Mn before the final annealing preceding the hot-dip coating. The aluminium adhering to the steel strip prevents the surface of the latter from oxidising in the course of the annealing of the steel strip which takes place before the hot-dip coating. The layer of aluminium, acting after the fashion of a primer, then causes the coating produced by the hot coating to adhere firmly to the steel strip over its full area even when the steel strip itself does not provide the right prerequisites for this due to its alloyed nature. For this purpose, advantage is taken in the known method of the effect that a diffusion of the iron from the steel strip into the layer of aluminium takes places in the course of the annealing treatment which has to precede the hot coating. A metallic overlay, consisting substantially of Al and Fe, which is connected by a firm bonding mechanism to the substrate formed by the steel strip, thus builds up on the steel strip in the course of the annealing.
A different method of coating a high-manganese steel strip containing 0.35-1.05 wt. % of C, 16-25 wt. % of Mn and remainder iron plus unavoidable impurities is known from WO 2006/042931 A1. In this known method, the steel strip of the above composition is first cold-rolled and then recrystallisation annealed in an atmosphere which is reducing in relation to iron. The annealing parameters are selected in this case to be such that an intermediate layer which is substantially entirely composed of amorphous (FeMn) oxide comes into being on both sides of the steel strip, and in addition there comes into being an outer layer which is composed of crystalline Mn oxide, the thickness of the two layers being at least 0.5 μm. There is no longer any hot-dip coating following this. Instead, it is the layer of Mn oxide in combination with the layer of (FeMn) oxide which is intended to provide adequate corrosion protection.
Based on a similar principle is the method described in WO 2006/042930 (EP 1 805 341 B1), in which, by two successive annealing steps, a layer of iron and manganese mixed oxides is first produced on the high-manganese steel substrate and an outer layer comprising Mn mixed oxides is then produced on this first layer. The steel strip which has been coated in this way is then conveyed into a bath of molten metal. As well as zinc, this bath of molten metal contains in addition a quantity of aluminium which is sufficient to reduce the layer of MnO completely and the layer of (FeMn)O at least partly. The intention is, as a result, to obtain a layered structure in which three layers of FeMnZn and an outer layer of Zn can be identified.
Practical studies have shown that even in steel strip which has been precoated in such a complicated and expensive way there is not, in practice, the adhesion to the steel substrate which is required for cold forming. Moreover, the method known from WO 2006/042930 proves not to be sufficiently reliable in operation due to the reactions which take place in the bath of molten metal, which are hardly possible to control in practice.
Finally, there is known from DE 10 2006 039 307 B3 a method for the hot-dip coating of a steel substrate having high Mn contents in which, to produce on the steel strip a metallic protective layer which is substantially free of oxidic intermediate layers, the ratio % H2O/% H2 of the water content % H2O to the hydrogen content % H2 of the annealing atmosphere is set in such a way, as a function of the given annealing temperature Ta, that the ratio % H2O/% H2 is equal to or less than 8·10−15·x Ta3.529, where T is the annealing temperature. Underlying this stipulation is the finding that, if the annealing atmosphere is set in a suitable way, namely if its hydrogen content is set in a suitable way in relation to its dew point, the nature of the surface which the steel strip to be coated acquires in the course of the annealing is one which will ensure that the metallic protective coating which is then applied by hot-dip coating will adhere in the optimum way. The annealing atmosphere which has been set in this way has a reducing action on both the iron in the steel strip and on the manganese therein. The aim in this case is to avoid the formation of an oxide layer which would interfere with the adhesion of the molten coating to the substrate of high-manganese steel.
Practical studies have shown that flat steel products prepared by the known method explained above do behave well as far as wetting is concerned and do have adhesion of the Zn coating which is adequate for many applications. However, in the forming of flat steel products coated in this way into components, it has been found that detachments and cracking of the coating still occur when the amounts of deformation are high.
Also, the methods known from the prior art may have an adverse effect on the mechanical properties in the flat steel product, in particular when the process temperatures used are high. Moreover, economical operation which comes into line with environmental requirements is not possible with the existing processes.
Against this background, the object of the invention was to specify a method which allows flat steel products having high contents of Mn to be provided with a zinc coating providing protection against corrosion, in the case of which coating it is ensured that there is a further improvement in the adhesion of the coating to the steel substrate. The intention was also to provide a flat steel product in which the Zn coating, which is formed in any given case from zinc or a zinc alloy, adheres securely to the steel substrate even under large amounts of forming deformation.